The field of the invention is systems and methods for x-ray mammography. More particularly, the invention relates to systems and methods for estimating breast density from an x-ray mammogram.
Mammographic density, hereafter referred to as breast density, describes the relative amount of fibroglandular tissue in the breast compared to the total amount of breast tissue, which is mostly composed of fibroglandular tissue and fat or adipose tissue. Mammographic breast density has been identified as an independent risk factor for breast cancer, and studies have identified a 4-5 fold increase in risk for developing breast cancer in women with dense breast tissue versus women with less dense breast tissue (i.e., breast tissue with more fat). The addition of breast density quantification to mammographic examination has the potential to greatly improve the accuracy of breast cancer risk assessment, especially for those without hereditary or familial risk factors. The inclusion of accurate breast density measurements can also be potentially helpful for women by suggesting that other imaging modalities such as magnetic resonance imaging (“MRI”) or ultrasound be used for initial screening instead of mammography because mammography's accuracy is known to be reduced in women with very dense breasts. Because the reporting of breast density is required in some jurisdictions, there is a desire to provide an accurate and reproducible quantitative density measurement method that is simple to implement on conventional digital mammography machines. To date, nearly all work in measuring breast density has used film-screen mammograms.
Quantitative methods, such as computer-assisted planimetry, can be very reproducible and are the best validated method for association with breast cancer risk, but they generally require at least some manual intervention and, thus, are time-consuming to use. With the increased utilization of digital mammography, automated computerized measurement of breast density is now becoming widely available, but has not yet been well validated. One major advantage of newer software methods applied to digital mammograms is that pixel signal levels can be measured objectively, yielding information about the composition of the breast tissue (e.g., volumetric breast density). The use of digital mammograms will allow automation and reduce variability. Automated breast density measurements that are both reproducible and demonstrated to be accurate could be an important addition to breast cancer risk assessment.
The current gold standard quantitative method of measuring breast density is the 2D Cumulus program developed by Dr. Martin Yaffe from Toronto. This method is a computer-assisted thresholding technique similar to planimetry. First, a film-screen mammogram is digitized. The pixels representing the total breast area and those representing dense breast area are then defined by a radiologist through an interactive program. The 2D Cumulus program yields the percent area density of a breast. There are, however, several limitations of the 2D Cumulus program.
The 2D Cumulus program uses binary information and a two-dimensional image of the breast. The binary nature of the procedure means that each pixel is counted as representing either one hundred percent breast tissue or one hundred percent fat, with no ability to represent a mixture of the two tissue types, or to account for the height of the column of tissue above the pixel. Furthermore, simple thresholding methods, such as 2D Cumulus, may cause dense tissue not to be included in the thin periphery of the breast, or may treat fatty tissue as being dense in regions where the compressed breast is thicker than average. Use of the 2D Cumulus program is also cumbersome, as it requires a radiologist or a trained scientist to visually select the division between fat and breast tissue, a process that can take as long as one minute per image. Because of these limitations, 2D Cumulus has only been used in the research setting and not in clinical practice.
For a risk model to be useful in clinical practice, breast density measurement should be automated, reproducible, accurate, precise, and, ideally, measured on a continuous scale. This eliminates observer bias and provides maximal discrimination.
The work of Shepherd, et al., (as described in U.S. Pat. Nos. 6,516,045; 6,654,445; and 7,873,198) calculates mass density, which is related to, but not equivalent to mammographic density. The process described by Shepherd requires that calibration materials of one-hundred percent fat and one-hundred percent glandular materials be placed on the breast support, and also requires that radio-opaque markers be present in each image to enable thickness measurements.
Standard compression methods for mammography use a movable, semi-rigid clear plastic compression paddle. The breast is placed on a bottom breast platform that is flat, and the paddle is then lowered onto the breast, usually while the technologist is holding the breast in place to ensure proper tissue coverage in the image receptor's field of view. A significant patient concern in mammography is the discomfort the patient feels when the breast is compressed with sufficient force to spread out the breast tissues. The reasons for using such high compression include: (1) to make the breast thinner and thereby reduce patient radiation exposure; (2) to improve image quality by reducing the amount of scattered radiation; (3) to make the breast more uniform in thickness in the direction of the x-ray flux, leading to a more uniform exposure over the entire breast image; (4) to immobilize the breast during the x-ray exposure, thereby reducing image blurring; and (5) to bring breast tissues out from the chest wall into the exposure area and thus image more tissue. A problem with the calculation of volumetric breast density is the requirement that thickness must be known accurately in order to relate the attenuation to a given mammographic density. For instance, two millimeters of error in the path length on the breast could result in an error of five percent or more in the breast density measurement.
On most mammography machines, the paddles and readout systems are not designed to produce uniform compression. All paddles show some deflection when compressed on a breast, often as much as 3 mm in the centre. They also deflect from front-to-back due to flexion of the mechanical components, which varies with the compression force. Some compression paddles also tilt, further introducing the possibility for variations in compressed breast thickness.
The electronic readout of thickness can be incorrectly calibrated, or the error may change with compression force applied. Even for those mammographic units that report thickness compensated for compression force, there can be errors as large as two millimeters. These errors can be much greater if the breast is not centered on the breast support plate. Calibrating for variations in thickness is time consuming, and not always accurate. As noted above, accurate measurement of compressed breast thickness is an important factor in determining breast density; however, the measurement of thickness provided by commercial mammography systems can differ by as much as one centimeter from the actual thickness due to deflection of the breast compression plate and the inaccuracies in the readout system.
Therefore, there remains a need to provide a system and method for quantifying volumetric breast density that can more accurately determine the thickness of the breast, thereby addressing the drawbacks of currently available methods.